» Corrosion Resistance of Titanium

The environmental resistance of titanium depends primarily on
a very thin, tenacious and highly protective surface oxide
film. Titanium and its alloys develop very stable surface
oxides with high integrity, tenacity and good adherence. The
surface oxide of titanium will, if scratched or damaged,
immediately reheal and restore itself in the presence of air or
water.

Titanium's already wide range of applications can be expanded by alloying with
certain noble elements or by impressed anodic potentials (anodic
protection).

Titanium is immune to corrosive attack by saltwater or marine
atmospheres. It also exhibits exceptional resistance to a broad
range of:

Also titanium generally exhibits superior resistance to
chlorides and various forms of localized corrosion. Titanium
alloys are considered to be essentially:

immune to chloride

pitting and

intergranular attack and

are highly resistant to crevice and stress corrosion

Another major benefit to the designer is the fact that
weldments, heat affected zones and castings of many of the
industrial titanium alloys exhibit corrosion resistance equal to
their base metal counterparts. This is attributable to the metal
lyrical stability of the leaner titanium alloys and the similar
protective oxide which forms on titanium surfaces despite
microstructural differences.

» Chlorine,
Chlorine Chemicals and Other Halogen Compounds

Titanium alloys are highly resistant to wet (aqueous)
chlorine, bromine, iodine and other chlorine chemicals because of
their strongly oxidizing natures. Titanium's outstanding
resistance to aqueous chlorides has been the primary historical
incentive for utilizing titanium in industrial service. In many
chloride and bromide-containing environments, titanium has
cost-effectively replaced stainless steels, copper alloys and
other metals which have experienced severe localized corrosion
and stress corrosion cracking.

» Chlorine Gas

Titanium is widely used to handle moist or wet chlorine gas,
and has earned a reputation for outstanding performance in this
service. The strongly oxidizing nature of moist chlorine
passivates titanium resulting in low corrosion rates. Proper
titanium alloy selection offers a solution to the possibility of
crevice corrosion when wet chlorine service temperatures exceed
155°F. (70°C.).

Dry chlorine can cause rapid attack of titanium and may even
cause ignition if moisture content is sufficiently low. However,
as little as one percent water is generally sufficient for
passivation or repassivation after mechanical damage to titanium
in chlorine gas under static conditions at room temperature.

» Chlorine Chemicals and Chlorine Solutions

Titanium is fully resistant to solutions of chlorites,
hypochlorites, chlorates, perchlorates and chlorine dioxide. It
has been used to handle these chemicals in the pulp and paper
industry for many years with no evidence of corrosion .

Titanium is used in chloride salt solutions and other brines
over the full concentration range, especially as temperatures
increase. Near nil corrosion rates can be expected in brine media
over the pH range of 3 to 11. Oxidizing metallic chlorides, such
as FeCI3, NiCI2, or CuCI2, extend titanium's passivity to much
lower pH levels.

A possible limiting factor of titanium alloy application in
aqueous chlorides can be crevice corrosion in metal to metal
joints, gasket to metal interfaces or under process stream
deposits. Given these potential crevices in hot chloride
containing media, localized corrosion of unalloyed titanium and
other alloys may occur depending on pH and temperature.

Service and laboratory-based guide lines, shown in Figure 1,
have been developed to aid in proper alloy selection. These
guidelines apply to most types of chloride solutions over a wide
range of salt concentrations. As indicated in Figure 1, the Grade
l2 and 7 titanium alloys offer improved crevice corrosion
resistance when solution pH increases or temperature increases.

» Halogen Compounds

Similar considerations generally apply to other halogens and
halides compounds. Special concern should be given to acidic
aqueous fluorides and gaseous fluorine environments which can be
highly corrosive to titanium alloys.

» Other Salt Solutions

Titanium alloys exhibit excellent resistance to practically
all salt solutions over a wide range of pH and temperatures. Good
performance can be expected in sulfates, sulfites, borates,
phosphates, cyanides, carbonates, and bicarbonates.

Similar results can be expected with oxidizing anionic salts
such as nitrates, molybdates, chromates, permanganates, and
vanadates; and also with oxidizing cationic salts, including
ferric, cupric, and nickelous compounds.

» Resistance to Waters

» Fresh Water/Steam

Titanium alloys are highly resistant to water, natural
waters and steam to temperatures in excess of 570°F
(300°C.) Excellent performance can be expected in high
purity water, fresh water and body fluids. Typical
contaminants found in natural water streams, such as iron and
manganese oxides, sulfides, sulfates, carbonates and
chlorides do not compromise titanium's performance. Titanium
remains totally unaffected by chlorination treatments used to
control biofouling.

» Seawater

Titanium is fully resistant to natural seawater regardless
of chemistry variations and pollution effects (i.e.
sulfides). Twenty year corrosion rates well below .01 mpy
have been measured on titanium exposed beneath the sea, in
marine atmospheres, and in splash or tidal zones. In the sea,
titanium alloys are immune to all forms of localized
corrosion, and withstand seawater impingement and flow
velocities in excess of 100 ft/sec (0.0003 mm/y). (See Table
1). Abrasion and cavitation resistance of these alloys is
outstanding, explaining why titanium provides total
reliability in many marine and naval applications. In
addition, the fatigue strength and toughness of many titanium
alloys are unaffected in seawater and lean titanium alloys
are immune to seawater stress corrosion.

Titanium tubing has been used with great success for more
than 20 years in seawater-cooled heat exchangers in the
chemical, oil refining and desalination industries. The
pH-temperature guidelines for crevice corrosion presented in
Figure 1 are generally applicable to seawater service as
well.

When in contact with other metals, titanium alloys are not
subject to galvanic corrosion in seawater. However, titanium
may accelerate attack on active metals such as steel,
aluminum, or copper alloys. The extent of galvanic corrosion
will depend on many factors such as anode to cathode ratio,
seawater velocity and seawater chemistry. The most successful
strategies eliminate this galvanic couple by using
more-resistant compatible, passive metals with titanium,
all-titanium construction, or dielectric (insulating) joints.
Other approaches for mitigating galvanic corrosion have also
been effective: coatings, linings and cathodic protection.

» Resistance to Acids

» Oxidizing Acids

In general, titanium has excellent resistance to oxidizing
acids, such as nitric and chromic, over a wide range of
temperatures and concentrations.

» Nitric Acid

Titanium is used extensively for handling nitric acid in
commercial applications. Titanium exhibits low corrosion
rates in nitric acid over a wide range of conditions (Table
2). At boiling temperatures and above, titanium's corrosion
resistance is very sensitive to nitric acid purity.
Generally, the higher the contamination and the higher the
metallic ion content of the acid, the better titanium will
perform. This is in contrast to stainless steels which are
often adversely affected by acid contaminants. Since
titanium's own corrosion product (Ti+4) is highly inhibitive,
titanium often exhibits superb performance in recycled nitric
acid streams such as reboiler loops.

One user cites an example of a titanium heat exchanger
handling 60% HNO3 at 380°F. (193°C.) and 300 psi (2.1 MPa)
which showed no signs of corrosion after more than two years
of operation. Titanium reactors, reboilers, condensers,
heaters and thermowells have been used in solutions
containing 10 to 70% HNO3 at temperatures from boiling to
600°F. (315°C.).

» Red Fuming Nitric Acid

Although titanium has excellent resistance to nitric acid
over a wide range of concentrations and temperatures, it
should not be used with red fuming nitric acid because of the
danger of pyrophoric reactions. Minimum water content and
maximum NO2 concentration (NO2/NO ratio) guidelines for
avoiding pyrophoric reactions in this particular acid have
been developed.

» Reducing Acids

Titanium alloys are generally very resistant to mildly
reducing acids, but can display severe limitations in
strongly reducing acids. Mildly reducing acids such as
sulfurous acid, acetic acid, terepthalic acid, adipic acid,
lactic acid and many organic acids generally represent no
problem for titanium over the full concentration range.

Common potent inhibitors for titanium in reducing acid
media include dissolved oxygen, chlorine, bromine, nitrate,
chromate, permanganate, molybdate and cationic metallic ions,
such as ferric (Fe+3), cupric (Cu+2), nickelous (Ni+2) and
many precious metal ions. Figure 2 shows how the useful
corrosion resistance of unalloyed titanium is significantly
extended as the ferric ion concentration is increased in very
small amounts. It is this potent metal ion inhibition
phenomenon which permits titanium to be successfully utilized
for equipment handling hot HCI and H2SO4 acid solutions in
metallic ore leaching processes.

Although inhibition is possible in most reducing acids,
protection of titanium from hydrofluoric acid solutions is
extremely difficult to achieve. Hydrofluoric acid will
generally cause rapid general corrosion of all titanium
alloys, and should, therefore, be avoided.

Since the presence of minute quantities of these common
inhibitive species can radically influence titanium's
performance in reducing acids, one must consider all details
of environment chemistry in the alloy selection process. Back
ground process stream species are frequently beneficial for
titanium. In addition, intentional addition of inhibitive
species to the process stream can be a practical approach for
extending titanium's corrosion resistance under marginal
conditions.

» Alkaline Media

Titanium is generally highly resistant to alkaline media
including solutions of sodium hydroxide, potassium hydroxide,
calcium hydroxide, magnesium hydroxide and ammonia hydroxide. In
the highly basic sodium or potassium hydroxide solutions,
however, useful application of titanium may be limited to
temperatures below 176°. (80°C.). This is due to possible
excessive hydrogen uptake and eventual embrittlement of titanium
alloys in hot, strongly alkaline media.

Titanium often becomes the material of choice for alkaline
media containing chlorides and/or oxidizing chloride species.
Even at higher temperatures, titanium resists pitting, stress
corrosion, or the conventional caustic embrittlement observed on
many stainless steel alloys in these situations.

» Organic Chemicals

Titanium alloys generally exhibit excellent resistance to
organic media. Mere traces of moisture and/or air normally
present in organic process streams assure the development of a
stable protective oxide film on titanium.

Titanium is highly resistant to hydrocarbons,
chloro-hydrocarbons, fluorocarbons, ketones, aldehydes, ethers,
esters, amines, alcohols and most organic acids. Anhydrous
methanol is unique in its ability to cause stress corrosion
cracking of titanium alloys. However, addition of more than 1.5%
water is sufficient to eliminate this problem.

Titanium equipment has traditionally been used for production
of terepthalic acid, adipic acid and acetaldehyde. Acetic acid,
tartaric acid, stearic acid, lactic acid, tannic acids and many
other organic acids represent fairly benign environments for
titanium. However, proper titanium alloy selection is necessary
for the stronger organic acids such as oxalic acid, formic acid,
sulfamic acid and trichloroacetic acids. Performance in these
acids depends on acid concentration, temperature, degree of
aeration and possible inhibitors present. The grade 7 and 12
titanium alloys are often preferred materials in these aggressive
acids.

» Resistance to Gases

Oxygen and Air Titanium has excellent resistance to gaseous
oxygen and air at temperatures up to 700°F. (370°C.). Above
this temperature and below 840°F. (450°C.), titanium may form
colored surface oxide films which may thicken slowly with time.
Above 1000°F. (545°C.) or so, titanium alloys lack long-term
oxidation resistance and will become brittle due to the increased
diffusion of oxygen into the metal.

Titanium alloys are totally resistant to all forms of
atmospheric corrosion regardless of pollutants present in either
marine, rural or industrial locations.

» Nitrogen and Ammonia

Nitrogen reacts much more slowly with titanium than
oxygen. However, above 1400°F. (800°C.), excessive
diffusion of the nitride may cause metal embrittlement.
Titanium is not corroded by liquid anhydrous ammonia at
ambient temperatures. Moist or dry ammonia gas, or
ammonia-water (NH40H) solutions will not corrode titanium to
their boiling point and above.

» Hydrogen

The surface oxide film on titanium acts as a highly
effective barrier to hydrogen penetration which can only
occur when this protective film is disrupted mechanically or
broken-down chemically or electro-chemically. The presence of
moisture effectively maintains the oxide film inhibiting
hydrogen absorption up to fairly high temperatures and
pressures. On the other hand, pure, anhydrous hydrogen
exposures should be avoided particularly as pressures and/or
temperatures increase.

Excessive absorption of hydrogen in titanium alloys leads
to embrittlement if a significant quantity of the brittle
titanium hydride phase precipitates in the metal. Generally,
hydrogen contents of at least several hundred ppm are
required to observe significant embrittlement.

The few cases of hydrogen embrittlement of titanium
observed in industrial service have generally been limited to
situations involving high temperature, highly alkaline media;
titanium coupled to active steel in hot aqueous sulfide
streams; and where titanium has experienced severe very
prolonged cathodic charging in seawater.

» Sulfur-Bearing Gases

Titanium is highly corrosion resistant to sulfur-bearing
gases, resisting sulfide stress corrosion cracking and
sulfidation at typical operating temperatures. Sulfur dioxide and
hydrogen sulfide, either wet or dry, have no effect on titanium
as shown in Table 4. Extremely good performance can be expected
in sulfurous acid even at the boiling point. Field exposures in
FGD scrubber systems of coal-fired power plants have similarly
indicated outstanding performance of titanium (see Table 5).

Wet S03 environments may be a problem for titanium in cases
where pure strong, uninhibited sulfuric acid solutions may form,
leading to metal attack. In these situations, the background
chemistry of the process environment is critical for successful
use of titanium.

» Corrosion
Resistance of High Strength Titanium Alloys

Most of the higher strength titanium alloys will exhibit
excellent resistance to general corrosion and pitting corrosion
in near-neutral environments which are neither highly oxidizing
nor highly reducing. The metallurgical condition of the alloy
plays a relatively minor role in corrosion performance, since
oxide film stability is assured in these situations. When severe
crevices exist in hot aqueous chloride media, most high strength
titanium alloys will exhibit slightly reduced crevice corrosion
resistance compared to unalloyed titanium. The titanium alloys
rich in molybdenum, however, exhibit excellent resistance to this
form of attack. It is for this reason, along with its favorable
strength and low density, the high molybdenum 3AI-8V 6Cr-4Zr-4Mo
alloy is a prime candidate for high temperature sour oil well and
geothermal brine well production tubulars and other downhole
components.

General corrosion resistance of high strength titanium alloys
in strongly oxidizing or strongly reducing environments may
diminish as aluminum and/or vanadium alloy content increases.
Improvements in resistance to hot reducing acids can be achieved
by increased alloy molybdenum content. The effects of various
alloying elements on titanium alloy corrosion have been studied
which indicate that suitable high strength alloys can be selected
for a wide range of aggressive environments.

Another major consideration is resistance to stress corrosion
cracking (SCC). Although the common industrial grades of titanium
are generally immune to chloride SCC, certain high strength
alloys may exhibit reduced toughness (Klc) values and/or
accelerated crack growth rates in halide environments. Most of
these alloys will not exhibit any susceptibilities to SCC in smooth
or notched conditions. However, above 480°F. (250°C.),
resistance to hot salt SCC should be considered if chlorinated
solvents, chloride salts, or other chlorine-containing compounds
contact component surfaces.

In summary, although high strength titanium alloys possess
corrosion resistance which is generally superior to that of most
common engineering alloys, consideration should be given to
selecting a titanium alloy with full compatibility to a given
environment. With the wide family of titanium alloys commercially
available today, optimum high strength titanium alloy selection
is almost always possible for a given environment. Technical
consultation is available to assist the designer/user in
achieving this end.

Therefore, in contrast to copper alloys, titanium piping,
equipment and heat exchangers can be designed for high flow
velocities with little or no detrimental effects from turbulence,
impingement or cavitation. This has positive implications
relative to optimizing heat transfer efficiency, minimizing
equipment wall thicknesses, minimizing tube and piping size and
wall requirements, improving equipment reliability and reducing
life cycle costs. It is for these reasons that titanium alloys,
in either wrought or cast form, have become prime materials for
various coastal chemical and power plants and marine/naval
applications.

» Titanium's oxide film

The protective passive oxide film on titanium (mainly TiO2) is
very stable over a wide range of pH, potential and temperature
and is especially favored as the oxidizing character of the
environment increases. For this reason, titanium generally
resists mildly reducing, neutral and highly oxidizing
environments up to reasonably high temperatures. It is only under
highly reducing conditions where oxide film breakdown and
resultant corrosion may occur.

This substantially inert surface oxide has high integrity and
tenacity. The oxide will, if scratched or damaged, immediately
restore itself in the presence of air or water. The film is
stable over a wide range of pH, electro-potentials and
temperature, particularly in oxidizing, neutral and mildly
reducing environments.

Titanium alloys are metallurgically stable and the protective
oxide forms equally on all titanium surfaces, on wrought
products, welds and castings irrespective of composition or
micro-structural differences.